Wireless sensors can be implanted within the body and used to monitor physical conditions, such as pressure or temperature. For example, U.S. Pat. No. 6,111,520 and U.S. Pat. No. 7,245,117 describe wireless sensors that can be implanted within the body and certain techniques that can be employed to communicate with the wireless sensors. These sensors can be used to monitor physical conditions within the heart or an abdominal aneurysm. An abdominal aortic aneurysm (AAA) is a dilatation and weakening of the abdominal aorta that can lead to aortic rupture and sudden death. In the case of a repaired abdominal aneurysm, a sensor can be used to monitor pressure within the aneurysm sac to determine whether the intervention is leaking. In the case of chronic heart failure, a pressure sensor, implanted in the right ventricle of the heart, can provide warning signs, in some cases up to five days in advance, before the patient experiences any external symptoms.
Typically, the implanted sensors utilize an inductive-capacitive (“LC”) resonant circuit with a variable capacitor. The capacitance of the circuit may vary with the pressure of the environment in which the sensor is located and thus, the resonant frequency of the circuit may vary as the pressure varies. The resonant frequency of the circuit can be tracked and used to calculate systolic, diastolic, and mean pressure. The resonant frequency can be obtained, or otherwise monitored, wirelessly using an antenna to obtain signals from the LC circuit. In some systems, an electronic system sends signals to the antenna for transmission to the LC circuit to energize or otherwise excite the LC circuit. The LC circuit responds with a signal that is received by the antenna and provided to the electronic system for analysis.
The electronic box, or system, is connected to the antenna by an antenna cable and, optionally, an antenna box or board that includes electronics used to operate the antenna. The cable may be a custom multi-wire cable assembly that includes different individual cables, each carrying a signal between the electronic box and the antenna. Signals carried by the multi-wire cable can include three power signals, a ground signal, a differential transmit gate signal, a differential receive gate signal, a transmit signal, and a receive signal. The three power signals can include a first positive voltage supplying power to the antenna box electronics, a second positive voltage supplying power for low voltage electronics in the antenna box, and a negative voltage supplying power for the low voltage electronics in the antenna box. The differential transmit gate signals are control signals for the transmitter functions in the antenna box electronics. The differential receive gate signals are control signals for the receiver functions in the antenna box electronics. The transmit signal is a signal that is transmitted by the antenna. The receive signal is a signal from the LC circuit that is received by the antenna. In some systems, the antenna cable may include ten different cables, such as two coaxial cables for the transmit and receive signals and eight single line wires for other signals, to provide the signals needed to control, power, and otherwise permit the antenna and the antenna box electronics to operate.
A multi-wire antenna cable system that includes ten different cables may be useful in some circumstances. FIGS. 1 and 2 illustrate an example of an antenna 10 that can be connected to an electronic box (not shown) via a relatively thick cable 12. The cable 12 may include multiple wires for carrying various types of signals between the antenna 10 and electronic box. For example, the cable 12 can carry transmitted signals from the electronic box, received signals from an implanted sensor, timing signals to control electronics associated with the antenna, and power to the circuitry in the antenna box. Each wire may carry one type of signal and the cable 12 may include ten to twelve, or more, different wires to provide such capacity. In some circumstances, the multi-wire antenna cable 12 may limit movement of the antenna 10 or antenna positioning—preventing the antenna 10 from being placed in an optimal position for obtaining signals from the LC circuit. Accordingly, a need exists for an antenna with increased placement flexibility for wirelessly communicating with an implanted sensor.